^^NSMBU Deluxe just added an extra powerup & pallet-swapped a Toad who can use it; everything else was already present in NSMBU/NSLU. Port.
SMM 2 is adding an entirely new mode of play (3D Land/Cat Bell), as well as new mechanics added to the previous modes. Sequel.

yeah the development of the series in inherently iterative, its hard to take something like "mario maker" and go in a completely different direction with it.

that said, considering how much new content is in the game that drastically alters the experiences you will have, as well as having an entirely redesigned creation interface, i would totally feel comfortable calling this a sequel